Process for stripping nitrogen oxides and sulphur oxides as well as optionally other noxious elements of flue gas from combustion plants

Abstract

Process for stripping nitrogen oxides and sulphur oxides, as well as optionally other noxious elements from flue gas from combustion plants, preferably boilers of electric plants supplied with fossil fuels, the nitrogen oxides being reduced into elemental nitrogen and the sulphur oxides as well as other noxious elements being stripped by a washing process. In order to improve this process of the above-mentioned type as to the simplicity of its implementation and the possible results, the present invention proposes a series of measures such as the addition of a monobasic and/or multibasic carbonic acid to EDTA with limestone or the like, the addition of phosphonic acid or the salt thereof to the washing liquid up to an electrochemical process whereby are added to the washing liquid of a conventional desulphuration plant salts which form with the nitrogen monoxide complexes which are reduced into elementary nitrogen and water by mixing them with formic acid to prevent the precipitation of iron-III-hollow hydroxide at a potentiostatic cathode.

We claim: 
     
       1. A process for stripping nitrogen and sulfur oxides from a fossil-fuel combustion gas, comprising the steps of: (a) scrubbing said combustion gas in at least one scrubbing stage with an aqueous scrubbing liquid;   (b) adding to said scrubbing liquid ethylenediaminetetraacetic acid to chelate iron, at least one other carboxylic acid, and at least one further substance selected from the group which consists of lime, limestone, fine white chalk, hydrated lime, phosphonic acid and phosphonic acid salts whereby a chelated iron nitrogen oxide complex is formed in said liquid; and   (c) reducing nitrogen oxides absorbed in said liquid and in the form of said chelated iron nitrogen oxide complex to elemental nitrogen by the steps of: (c 1 ) adding a reducing agent consisting at least of sodium dithionite to said aqueous scrubbing liquid, and   (c 2 ) potentiometrically reducing said chelated iron nitrogen oxide complex to elemental nitrogen and water between an anode and a cathode in contact with said liquid.     
     
     
       2. The process defined in claim 1 wherein phosphonic acid or a phosphonic acid salt is added to said scrubbing liquid in step (b) in addition to said ethylenediaminetetraacetic acid. 
     
     
       3. The process defined in claim 1 wherein scrubbing of said combustion gas is effected in two stages including a first stage with said scrubbing liquid to which said ethylenediaminetetraacetic acid is added in step (b) at a pH of 2.5 to 4.5, hydrochloric acid being added to said liquid in said first stage, a second scrubbing stage being effected for the desulfurization of said combustion gas. 
     
     
       4. The process defined in claim 1 wherein said combustion gas is scrubbed in a plurality of stages including a first hydrochloric acid scrubbing stage, a second stage in which said combustion gas is scrubbed in accordance with steps (a) and (b), and a third stage for desulfurization of said combustion gas. 
     
     
       5. The process defined in claim 1, further comprising the step of recovering ethylenediaminetetraacetic acid from said scrubbing liquid by reducing the pH thereof to substantially 2.0. 
     
     
       6. The process defined in claim 1, further comprising the step of complexing the ethylenediaminetetraacetic acid added in step (b) to said scrubbing liquid with iron-II sulfate before the ethylenediaminetetraacetic acid is added to said scrubbing liquid. 
     
     
       7. The process defined in claim 6, which further comprises the step of acidifying said scrubbing liquid with sulfuric acid, simultaneously with the addition of iron-II sulfate thereto. 
     
     
       8. The process defined in claim 6, further comprising the step of raising the pH of said scrubbing liquid by adding thereto an alkali simultaneously with the addition of iron-II sulfate. 
     
     
       9. The process defined in claim 6, further comprising the step of adding to the scrubbing liquid in step (b) at least one phosphonic acid complex with iron-II. 
     
     
       10. The process defined in claim 1 in which nitrogen oxides are reduced in step (c) at least in part by adding to said scrubbing liquid an alkali or alkaline earth bisulfite produced in a prior scrubbing stage of said combustion gas. 
     
     
       11. The process defined in claim 1 wherein in step (c) said reducing agent includes ascorbic acid. 
     
     
       12. The process defined in claim 1 wherein nitrilotriacetic acid is added to said scrubbing liquid. 
     
     
       13. The process defined in claim 1 wherein in step (c) metallic iron is added to said aqueous scrubbing liquid at a pH less than 6. 
     
     
       14. The process defined in claim 13 wherein iron and sodium dithionite are added in separate steps to the scrubbing liquid in step (c). 
     
     
       15. The process defined in claim 1 wherein said sodium dithionite is formed electrochemically in situ within said aqueous scrubbing liquid. 
     
     
       16. The process defined in claim 1 wherein in step (c 2 ) chlorine is formed on said anode and is removed, metal separation occurs on said cathode from which the separated metals are removed, and the anode is provided in an anode cell isolated by a membrane and in which the nitrate ion enrichment occurs. 
     
     
       17. The process defined in claim 16 wherein said membrae is an anion exchanger material, said anode is titanium and said cathode is carbon. 
     
     
       18. The process defined in claim 16 wherein oxidizing gases formed on said anode are used to oxidize sulfite to sulfate in the scrubbing of said combustion gas. 
     
     
       19. A process for stripping nitrogen and sulfur oxides from a fossil-fuel combustion gas, comprising the steps of: (a) scrubbing said combustion gas in at least one scrubbing stage with an aqueous scrubbing liquid;   (b) adding to said scrubbing liquid ethylenediaminetetraacetic acid to chelate iron, at least one other carboxylic acid, and at least one further substance selected from the group which consists of lime, limestone, fine white chalk, hydrated lime, phosphonic acid and phosphonic acid salts whereby a chelated iron nitrogen oxide complex is formed in said liquid; and   (c) reducing nitrogen oxides absorbed in said liquid and in the form of said chelated iron nitrogen oxide complex to elemental nitrogen by potentiometrically reducing said chelated iron nitrogen oxide complex to elemental nitrogen and water between an anode and a cathode in contact with said liquid.